Conventionally, known automatic transmissions are made to remain in an already functioning gear ratio while coming to a stop. In detail, this means that the power connection, between the vehicle motor and the drive wheels, remains in a current state, until the vehicle comes to a stop. Following a preselected dwell time (for example, this being two seconds after the vehicle stops), due to additional conditions made available during a stop, where a gas pedal is not activated and the vehicle is motionless, the transmission shifts into the above mentioned standby mode. In this standby mode, the transmission can transfer very little torque.
When stopping a motor vehicle with such an automatic transmission, the following driving conditions play a role.
Upon braking the vehicle, in many cases, a mechanical clutch arrangement, which bypasses the hydrodynamic converter, is disengaged with the result that input torque is now passed through the converter. As this occurs, the converter must deal with the idling engine running, where the torque passed to the drivetrain must now travel through the converter. At this timer the converter is in an idling operation and as the torque from the drive wheels of the previously rapidly traveling vehicle is now being transferred in the reverse direction through the converter to the motor, which is now idling. If the forward speed of the vehicle further declines, a point is reached, wherein the rotational speed of the converter turbine (hereinafter “turbine”), which is coupled with the drive wheels by the transmission, is approximately the same as the rotational speed of the converter pump (hereinafter “pump”) which is directly bound to the motor. In this equality of rotational speeds, the power train is torque-free.
If the vehicle is additionally braked and thereby the rotational speed of the turbine is further diminished, then the converter must cope with the idling speed. This means that the motor, limited to a rotational speed by during idling, receives torque from the converter and the transmission, which is generated by the normally driven wheels. The effects of which must be braked by the service brake. Under these conditions, a downshift of the transmission, most likely from the second gear ratio into the first gear ratio (RS 2-1) must be carried out, where the tractional torque is again reduced. If the vehicle continues to slow down, a resurgence in tractional torque suddenly occurs so that the vehicle must be more strongly braked again with the service brakes to offset this torque transmitted into the idling motor through the transmission and the converter. The attendant result brings about an unnecessary and wasteful consumption of fuel.
Introduced into the converter, the torque converts itself to wasteful heat, practically to the extent of 100%. This accumulation of heat appears during driving by way of problems such as unfavorable cooling conditions during travel; a reduced throughput of oil in the transmission; diminished cooling water circulation, and a reduction of cooling air.
An additional problem can be seen therein, in that, as a result of tractional torque being developed by the motor being sent through the converter to the drive wheels, the braking capability at the powered axle is diminished and the respective other axle can develop a tendency of being locked.
As has already been described and explained, after stopping the vehicle and when additional stopping conditions are in force, the transmission necessarily follows a preselected dwell time and shifts into the standby mode. If the vehicle stops, during a climb, the driver has a tendency to release pressure on the brake pedal, since the vehicle is at first held in place, being partially powered by the residual drive torque of the motor, which is transmitted to the driving wheels. If now, the transmission shifts into the standby mode, this shift is itself evident by a release impact, which is unpleasantly detected by the driver. Further, at this moment, the drivetrain becomes torque-free so that the driver, under certain circumstances, then presses even harder on the brakes in order to prevent a rolling of the vehicle.
With this above stated background, the present invention proposes a method, where the above described disadvantages are avoided. These disadvantages are ineffective fuel consumption, poor cooling, during both the stopping process and the period of standstill, the undesirable release impact and unrestrained, downhill back-rolling of the vehicle.